1. Field of the Invention
This invention relates to a thermally-assisted recording (TAR) type of magnetic recording hard disk drive that uses patterned media, wherein each data bit is stored in a magnetically isolated island on the disk, and more particularly to TAR disk drive with patterned multilevel media.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media, also called bit-patterned media (BPM), have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data islands or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the regions between the islands must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is substantially no magnetic material in the regions between the islands.
A problem with magnetic recording materials used for both continuous and BPM is low thermal stability. As the magnetic grains become smaller to achieve ultrahigh recording density they become more susceptible to magnetic decay, i.e., magnetized regions spontaneously lose their magnetization, resulting in loss of data. This is attributed to thermal activation of the small magnetic grains and is called the superparamagnetic effect. The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic recording material and V is the volume of the magnetic grain. Thus recording material with a high Ku is important for thermal stability, although the Ku cannot be so high as to prevent writing by the magnetic write field from the write head. To enable the use of high Ku (high anisotropy) material, heat-assisted magnetic recording (HAMR), also called thermally-assisted recording (TAR), has been proposed. In TAR systems, an optical waveguide with a near-field transducer (NFT) directs heat from a radiation source, such as a laser, to heat localized regions of the magnetic recording layer on the disk. The radiation heats the magnetic material locally to near or above its Curie temperature to lower the coercivity enough for writing to occur by the write head. TAR systems have also been proposed for disk drives with BPM, wherein each data island is heated simultaneously with the application of the write field from the write head.
Multilevel BPM has been proposed, wherein each data island has multiple stacked magnetic cells that are magnetically decoupled from each other by nonmagnetic spacer layers. In multilevel BPM the data density is increased by a factor of 2(n-1), where n is the number of cells in each island. U.S. Pat. No. 6,865,044 B1, assigned to the same assignee as this application, describes a disk drive with multilevel BPM having two magnetic cells in each data island, wherein the upper cell is written to in the conventional manner with a magnetic write field, and both the upper and lower cells are written to using TAR by heating the cells. However, this type of disk drive suffers from the problem of bit addressability because the write field or fringe fields can overwrite data in the upper cells of the closely-spaced data islands near the data island being written to. The overwriting is more likely to occur because patterned perpendicular media may have a wide distribution of the switching field, i.e., the write field required to switch the magnetization of an island from one magnetic state to the other state. Since the magnetic field gradients of the recording head are finite, the residual magnetic field on the neighboring islands can cause unintentional overwriting of the upper cells on these islands. BPM recording with TAR alleviates this problem because the physical separation among islands reduces lateral heat flow and provides a larger effective write gradient. Additionally, because the upper cells must have a coercivity low enough to be written to by a conventional write head, they cannot be formed of high anisotropy material, which is desirable to achieve media with high thermal stability.
What is needed is a disk drive with multilevel BPM that does not suffer from the problems of bit addressability and low thermal stability.